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So, there is no rewards in circulation when older posts are voted, right?

That's correct, rewards are unaffected.

Does it affect your voting power, though, when you upvote old posts, or can you go around and upvote millions of old posts at full power without it diluting your ability to vote on new content?

Yes, it does cost voting power.

If it does cost voting power - where then does the reward go that have been created?

What if an 8 day old post gets a new comment. Does upvoting the new comment in an old post result in curation reward?

It will result in a 25% curation reward for the user who voted that comment, and a reward for the person who wrote the comment.

Does the comment have its own reward window? Is it separate from the reward window for the post (that already ended)?

Yes, the reward window is separate at it pays out 7 days after the comment is made.

Now I understand a lot more about the curation reward process. Previously, it deterred me from voting and commenting on posts older than 7 days.